The steady increase of capacity demand in transport core networks requires cost-effective solutions for increasing the amount of traffic that can be carried by fibre infrastructures which have already been deployed. Current network architectures based on a single grid wavelength allocation scheme (e.g. 50 GHz or 100 GHz grids as defined by the International Telecommunication Union's Telecommunications Standardisation Sector, or ITU-T) can result in congestion of certain links and blocking in parts of a network, while spectral bandwidth is still available in other parts of the network. In order to accommodate an ever-growing capacity demand in a fibre-limited environment, it is therefore necessary to push network capacity beyond that allowed by standard wavelength grids.
This problem is compounded by the desire to keep the cost and energy consumption of the network to a minimum. Increasing transmission via transparent optical data path generally reduces the amount of energy consumption needed for optical transmission in the optical layer but may cause increased energy consumption in other/higher layers of the network. That is to say that the higher the degree of transparency, the fewer optical-electrical-optical (OEO) conversions will be present in the network. Several methods and systems have been developed to attempt to address this problem.
One such method involves the use of a uniform wavelength grid with smaller-than-50 GHz spacing, as used for instance in submarine transmission links to increase their spectral efficiency. In meshed networks, this solution requires changing all wavelengths selective switches (WSS) to ensure compatibility with the new spacing. In addition, the smaller spaced grid causes increased physical impairments (e.g. cross-talk and cross-phase modulation, or XPM) and forces the use of numerous OEO regenerators even for demands passing through links which are far from congestion.
Another known method consists of dividing the spectrum of each link into two or more bands, with different channel spacings on each band. The main drawback of such a solution is its lack of flexibility, because the spectrum of all links has to be split in bands of the same width regardless of their level of congestion to avoid fragmentation problems. As is the case with the previous solution, the use of narrow spacing where not needed reduces the transparent reach and increases the number of required OEO. Moreover, this solution is not compliant with most current WSS architectures.
Another known approach is to divide the spectrum of the network into a transparent band and an opaque band and use as close a spacing as physically feasible (depending on link length) in the opaque band. The channel spacing (and hence link capacity) can thus be adjusted on a link-per-link basis, avoiding wavelength continuity issues while retaining some degree of transparency. This approach however, wastes transparent bandwidth in uncongested links and opacity requires a high number of OEO resources. This makes it difficult to control the increase in price/energy consumption per connection for a target capacity increase.